Mills' father is a Torres Strait Islander from Thursday Island and his mother is an Aboriginal Australian from the Ynunga people of South Australia. He was first exposed to basketball as a four-year-old with The Shadows, an Indigenous Australian team that his parents established.[4] Mills attended Marist College Canberra, where he played a variety of sports in addition to basketball, such as Australian football and athletics, and served as ball boy for the Canberra Cannons of the NBL. During that time, his family developed a close relationship with Cannons' player David Patrick, who would eventually play a key role in Mills' career.[5]

As well as playing basketball, Mills also played underage Australian rules football at a high level. He represented the Australian Capital Territory at the 2004 Under-16 National Championships, and, after being named the tournament's best and fairest player, was pursued by recruiters from the Sydney Swans, an Australian Football League team.[6][7] At the age of 15, Mills was selected for the ACT's under-18 basketball squad to compete in the Australian National Championships. The following year, Mills accepted a full-time basketball scholarship with the Australian Institute of Sport. He replaced a player who had dropped out of the program, Scott Pendlebury, who had dropped out to (successfully) pursue an AFL career.[6] He also made a strong impression at the 2005 Australian Olympic Youth Festival, an event considered to be a showcase for future elite sporting talents.[4]

In January 2006, he received the RE Staunton Medal as top player in the Australia under-20 national championships.[8] In April that year, he received more international attention when he was named to the World team of under-20 players that played a US team of the same age group at the Nike Hoop Summit in Memphis, Tennessee. Mills led the World squad with 6 assists and also added 8 points in a losing effort.[9] He was one of 22 players invited to try out for the Australia squad for the 2006 FIBA World Championship, becoming the youngest player ever to participate in a Boomers training camp. While he was in training with the Boomers in July, he was named by Basketball Australia as the Junior Male Player of the Year.[8] Mills also had an outstanding 2006 season with the AIS men's team in the South East Australian Basketball League (SEABL), earning him a place on the All-SEABL East men's team and the honour of 2006 SEABL U-21 Australian Youth Player of the Year. He went on to be named the 2006 Basketball Australia Player of the Year, and also received two major awards from Indigenous Australian organisations—The Deadlys Award for Most Promising New Talent in Sport and NAIDOC's National Sportsperson of the Year.[10]

Mills signed with Saint Mary's College of California during the NCAA national early signing period in November 2006. Although he was pursued by several other NCAADivision I schools such as Utah (where Andrew Bogut played before coming to the NBA), Wake Forest, Alabama, and Nebraska (where he would have joined fellow Australian Aleks Marić), he came to Saint Mary's, indicating that he was comfortable with the Gaels' head coach Randy Bennett.[11] Also, just prior to the 2006–07 season, Bennett had hired Mills' old family friend, David Patrick, as an assistant coach.[5][12] Mills became the fifth Australian that Bennett successfully recruited since arriving at St. Mary's in 2001. One of the other Australians to have played under Bennett at Saint Mary's was fellow AIS old boy Daniel Kickert, the Gaels' all-time leading scorer.[10]

Mills also followed in Kickert's footsteps in another way, becoming the first freshman to start for the Gaels since Kickert did so in the 2002–03 season.[13] He first broke into the American basketball consciousness in his fourth game of the 2007–08 season, when he scored 37 points to lead the Gaels to an upset of nationally ranked Oregon on 20 November 2007.[14] The win also validated pre-season expectations for the Gaels, who were picked before the season to finish second in the West Coast Conference behind traditional conference power Gonzaga.[14] The Gaels went on to start the season 7–0 and earn their first national ranking since 1989.[15] The Gaels also won the 2007 edition of the Rainbow Classic, an annual eight-team December tournament hosted by the University of Hawaiʻi, with Mills being named tournament MVP.[16]

St. Mary's was nationally ranked for most of the season, ultimately finishing second in WCC regular-season play to Gonzaga. After the regular season, Mills was named WCC Newcomer of the Year and also made the All-WCC first team. At the end of the regular season, Mills was the second-leading freshman scorer in the WCC at 14.4 points per game, and also led all WCC freshmen in assists (3.47 per game) and steals (1.80 per game).[17] The Gaels went on to the NCAA tournament, where they lost in the first round to Miami (FL). In that game, Mills' 24 points made him the only Gael to score in double figures, and he also led the Gaels with 5 assists.[18] Despite the 19-hour time difference between Mills' hometown of Canberra and the U.S. West Coast, Australian television showed Gaels games live.[15]

In 2008–09, Mills was named to the All-WCC first team for the second year in a row after averaging 18.4 points, 2.4 rebounds, 3.9 assists and 2.2 steals per game.[19]

In April 2009, Mills entered the 2009 NBA draft, waiting until the final minutes of the deadline before making the decision.[20] He was selected with the 55th overall pick by the Portland Trail Blazers.

During practice for the NBA Summer League, he suffered a fracture in his right foot, and required surgery.[21] He signed with the Blazers in October 2009 after speculation of whether he would be signed at all due to the injury and the number of point guards already on the roster.[22]

After completing rehabilitation from surgery on his foot in December, Mills was assigned to the Idaho Stampede of the NBA Development League on December 29.[23] In his debut with Idaho, he scored 38 points and added 12 assists and 3 rebounds.[24] After one more game with the Stampede, in which he scored the game-winning basket,[25] Mills made his NBA debut for the Trail Blazers on 4 January 2010. In just under 5 minutes, he registered 2 assists but scored no points.[26] On 13 January 2010, he was again assigned to the Stampede, before being recalled ten days later.[27]

On 20 November 2011, Mills signed a non-guaranteed contract with the Xinjiang Flying Tigers of China, parting ways with the Tigers after nine games.[29][30] He finished his short NBL stint with averages of 18.6 points, 2.3 rebounds and 5.0 assists per game.[31] On 4 January 2012, he was released by Xinjiang for allegedly faking a hamstring injury.[32][33] In 12 games for Xinjiang, he averaged 26.5 points, 3.8 rebounds, 3.8 assists and 2.7 steals per game.

On 27 March 2012, Mills signed with the San Antonio Spurs.[34] On 26 April 2012, Mills set career highs with 34 points and 12 assists for his first NBA double-double in the Spurs' 107–101 win over the Golden State Warriors.[35] Mills also surpassed Andrew Bogut's record for most points in a single NBA game by an Australian.[36]

On 13 July 2012, Mills re-signed with the Spurs on a multi-year deal.[37] In 58 games, Mills averaged 5.1 points and 1.1 assists per game in 2012–13. Mills and the Spurs reached the 2013 NBA Finals against the Miami Heat, but they would lose the series in seven games. Mills was inactive for the last three games of the NBA Finals after having foot surgery.

On 24 June 2013, Mills exercised his player option to return to the Spurs for the 2013–14 season.[38] Mills averaged 10.2 points that season as the Spurs went on to win the NBA championship, defeating the Miami Heat 4-1 in the NBA Finals. In Game 5 of the Finals, Mills scored a playoff career-high 17 points, including 14 points in the third quarter, as the Spurs clinched the series.

Despite being ruled out for seven months due to a shoulder injury,[39] Mills re-signed with the Spurs on 12 July 2014.[40] He went on to recover quicker than expected as he made his return on 28 December 2014 in the Spurs' 110-106 win over the Houston Rockets. In just over 20 minutes of action, he recorded 8 points, 4 rebounds, 1 assist and 1 block.[41]

After repeating as Staunton Medallist in 2007,[42] Mills completed his scholarship with the AIS and opted to play for Saint Mary's. Before arriving in the Bay Area in August 2007, he played for Australia at the FIBA U-19 World Championship in Serbia, in which Australia finished fifth. He was then called up to the Boomers in July 2007, making his debut against Austria on 31 July in an international friendly tournament in Bormio, Italy. Mills scored 11 points off the bench and was credited with changing the complexion of the match, which Australia won 84–63.[43] This not only made him one of the youngest Boomers ever,[44] but also made him only the third Indigenous Australian, after Michael Ah Mat in the 1960s and Danny Morseu in the 1980s, to play for the Boomers.[1] Mills went on to make the Boomers squad for the FIBA Oceania Championship, which doubled as a qualifier for the 2008 Olympics. His main highlight in the competition, which was a three-match series against Australia's trans-Tasman rivals New Zealand, came in the second and ultimately deciding match. Mills came off the bench to score 17 points, helping lead the Boomers to a 93–67 rout of the Tall Blacks that punched their ticket to Beijing.[11]

Mills averaged a team-best 14.2 points, all off the bench, in the six games the Aussies played in Beijing. He scored 20 points in a quarterfinal loss to the United States, with three assists, two steals and zero turnovers in 28 minutes.[citation needed]

At the London International Basketball Invitational, the Boomers lost a tight contest to France, 71–67. Playing against NBA star Tony Parker, Mills scored a team high 20 points. The following game against Croatia, Mills led the Boomers to a 78–66 win, scoring a game high 22 points. Mills scored a tournament high 39 against Great Britain.[45]

During the 2010–11 NBA season, Mills and teammate Rudy Fernández created a trend in which certain Blazers formed the "A-OK" sign with each hand and placed them over their eyes after making a three-point shot. The gesture was meant to resemble a pair of goggles with three fingers in the air to reference the shot just made. When Fernández would struggle with his three-pointers, Mills used to joke that he needed help seeing. After knocking down threes, Fernández would put on the goggles as if to say, "I can see with these on." The trend quickly spread across the team, becoming routine for players such as Mills, Fernández, Wesley Matthews, Marcus Camby and Brandon Roy. T-Shirts with the gesture were printed and popularly sold in Portland.[46] Nuggets guard J. R. Smith performed the goggles after sinking a three-pointer against the Blazers, a gesture which irritated some in Portland but demonstrated the popularity of the gesture.[47] The gesture has caught on at many levels of basketball in America; for example, Darius Johnson-Odom, Doron Lamb and Nolan Smith adopted it in college to celebrate their baskets.[48]

^Mills is sometimes credited with being the youngest ever, but this is incorrect. Mills debuted on 31 July 2007, at the age of 18 years and 11 months. But the AIS Representative History page records that Andrew Vlahov, born 1 April 1969, debuted in 1987, so Vlahov could not have been older than 18 years 9 months.